Electro-thermal fluid displacement actuators that convert electrical energy into thermal energy and, in turn, employ the thermal energy to expand a thermally expandable fluid medium to do mechanical work are known. Examples of such actuators are described in commonly assigned U.S. Pat. Nos. 4,070,859, 4,079,589, 4,104,507 and 4,759,189. The disclosures of these patents are incorporated herein by reference.
In the type of electro-thermal actuator with which the invention is concerned, a working fluid is contained within a boiler chamber at one end of the actuator. Preferably, that fluid is liquid at room temperature. Upon being heated to a sufficient temperature, the working fluid changes from a liquid phase to a gas phase. The phase transition results in an increasing pressure within the boiler chamber so that some of the working fluid travels to a variable volume chamber within the actuator. The expanding working fluid presses against a diaphragm in the variable volume chamber, displacing the diaphragm which drives a piston that extends a piston rod from the actuator. The out-stroking motion of the piston may be employed to do work on or to engage an external device. A return spring mounted within the actuator biases the piston to retract the piston rod. As long as the pressure of the working fluid is sufficient to overcome the biasing force of the return spring, the piston rod remains extended from the actuator. When the working fluid cools, the pressure on the diaphragm is reduced and working fluid flows back into the boiler chamber. In response, the piston rod is retracted into the actuator by the biasing force applied by the return spring. The gaseous working fluid returns to the liquid phase when sufficiently cool.
In order to heat the working fluid and, preferably, to produce a phase change in the working fluid, an electrically driven heater is disposed within the boiler chamber. The heater may alternatively be a simple resistance heater that produces heat in response to a current flow through the heater. The heater may be a positive temperature coefficient (PTC) heating element that also is heated by a flow of electrical current through it. Since a PTC heating element significantly increases in resistance once it reaches a particular temperature, it inherently limits the magnitude of the steady state current that flows in response to a particular voltage applied to the heater. To improve actuator response time without excessive current flows, a resistance heater may be connected in series with a PTC heating element.
Each of the actuators described in the patents incorporated herein by reference includes a throttling-type valve disposed between the boiler and variable volume chambers. The valve comprises a rigid separator including one or more orifices through which the working fluid can pass. In these actuators, the diaphragm driving the piston assembly is usually pressed directly against the rigid separator by the return spring to close the orifices when the piston is retracted. That arrangement requires the force applied by the working fluid in the boiler chamber to equal that produced on the diaphragm by the return spring before the piston begins to move. This design provides a "snap-action" extension of the actuator piston rod.
In some applications excessive forces and vibrations are applied to the piston rod when it is in the extended position, i.e., when the working fluid is at a high pressure both in the boiler and variable volume chambers. When those external forces urge the extended piston rod toward its retracted position, large forces are applied to the rigid separator between the boiler and variable volume chambers. Those forces can shatter the separator. While the actuator can continue to function without an intact separator, the fragments from the shattering of the separator can puncture the diaphragm permitting the working fluid to escape. When the working fluid escapes, the operation of the actuator is severely impaired or destroyed.